The invention relates to a method for the forming of glass or glass ceramics, in particular for the preparation of mirror substrates, wherein a formed body is prepared from a blank by sagging under gravity force onto a mold.
The invention further relates to a method for preparing a mold suitable therefore.
From DE 199 38 811 A1 a method for the forming of glass ceramic parts is known according to which a blank is sagged under the influence of infrared irradiation, while possibly applying a vacuum or an excess pressure.
Such a forming method is suitable for forming a large number of blanks in three dimensions. Thus, e.g. trough-like or chamfer-like parts having a cross-section in circle segment form, rectangular, trapezoid or other form can be prepared.
The precision of the parts manufactured thereby naturally depends on the precision of the mold that is utilized. Up to now according to this method only mass-produced parts have been manufactured, while utilizing mostly metallic molds.
For the production of telescopes for scientific X-ray satellites up to now polished solid cylindric mirror shell substrates made of the glass ceramic ZERODUR® (glass ceramic, Schott Glas) have been utilized as projection optics. Due to the low reflectivity of all known materials for high-frequency X-rays at normal angle of incidence X-ray mirrors and X-ray telescopes are, preferably, operated at grazing incidence while taking advantage of the physical effect of total reflection. To this end two-piece telescopes for two reflexions are utilized, wherein the cylindric mirror shells have the specific forms of a parabola and a hyperbola which, according to Wolters, are particularly suited for such applications. For the telescopes of the scientific satellites ROSAT and CHANDRA polished solid cylindric mirror shell substrates of ZERODUR® (glass ceramic) were utilized as imaging optics. By contrast, for the X-ray satellite XMM-NEWTON galvanically produced nickel shells having a similar Wolters profile were utilized as mirror substrates. Up to now all X-ray satellites have applied conical solid cylindrical mirror substrates.
For the next generation of planned X-ray satellites (XEUS, CONSTELLATION-X) considerably larger telescopes are intended. Due to cost considerations such telescopes cannot be manufactured anymore as solid cylinders, but shall be composed of segmented mirror segments. Two different replicating methods for the mirror shells of these novel X-ray telescopes are currently intended to proceed from the single production of current X-ray mirrors to a line production and mass manufacturing for the planned satellites. Both methods utilize precision formed bodies of the glass ceramic ZERODUR® (glass ceramic), known as mandrels. The precision of the mandrel is transferred to the mirror shell segments by means of a nickel galvanic method (XEUS) or by means of duplicating utilizing an epoxy synthetic as an intermediate layer (CONSTELLATION-X). Both processes are performed at low temperatures below 100° C.
For the mentioned novel replicating processes for the manufacture of segmentized X-ray mirrors preformed glass substrates are necessary. This holds true for the planned project of NASA CONSTELLATION-X, as well as for the planned satellite of ESA XEUS, however, with respect to the latter only as an alternative technology for the favored galvanic method.
The mentioned mandrels of ZERODUR® (glass ceramic), are not suitable as molds for the manufacture of preformed glass substrates, since the operating range of ZERODUR® (glass ceramic), is 600° C. at the most, while already starting from 130° C. particular restrictions are present.
However, for a cost-effective molding of mirror substrates temperatures are necessary which, in part, are considerably above 600° C.
On the other hand, sintered ceramic molds cannot be manufactured in particular at large dimensions with the necessary form precision and precision (e.g. freeness from pores).
However, the manufacture of quartz molds is very expensive and restricted to smaller dimensions.
From EP 1,170,264 A1 it is basically known that glasses of the system Li2O—Al2O3—SiO2 can be transformed into glass ceramics (LAS glass ceramics) with high quartz mixed crystals and/or keatite mixed crystals as predominant crystal phases. These glass ceramics are prepared in various steps. After melting and hot forming usually the material is cooled below the glass transition temperature. Thereafter the base glass is transformed into a glass ceramic article by controlled crystallization. This ceramization is performed by an annealing process having several steps in which in the beginning nuclei are formed by nucleation at a temperature between 600 and 800° C., usually from TiO2 or ZrO2/TiO2 mixed crystals. Also SnO2 may take part in the nucleation. During a subsequent raise of temperature high quartz mixed crystals grow on these nuclei at a crystallization temperature of about 750 to 900° C. Herein the volume fraction between the crystalline high quartz mixed crystal phase and the glassy phase can be controlled in such a way that a coefficient of expansion of about 0 is reached. To this end normally a fraction of about 80% high quartz mixed crystals to about 20% residual glass is desired.
According to EP 1,170,264 A1 a short-time temperature increase up to 1100° C. or more is performed, whereby the glass ceramic is transformed into a ceramic having predominantly a keatite mixed crystal phase in the core and having a high quartz mixed crystal phase close to the surface.
However, the application of this glass ceramic disclosed herein is limited to cooking surfaces, cooking utensils, fire-proof glass etc.